Temperature Measurements Principles Measuring Devices Applications بِسْـــــمِ اللهِ الرَّحْمَنِ الرَّحيـــــِمِ

Temperature Measurements

PrinciplesMeasuring Devices

Applications

حيـــــم الر حمن الر الله بســـــم

Definition of Temperature• An expression for the kinetic energy of vibrating

atoms and molecules of matter.• Can be measured by various secondary

phenomena, e.g.,– change of volume or pressure, – electrical resistance, – electromagnetic force, – electron surface charge, or – emission of electromagnetic radiation.

BME 353 - Biomedical Measurements and Instrumentation

212 Oct. 2015

Direct and Indirect• Many engineering applications require direct

measurement of temperature. – Synthetic fuel research, – solar energy conversion and – new engine development are a few of these disciplines.– All industries place new emphasis on energy efficiency.

Hence, the fundamental measurement of temperature assumes new importance.

• Temperature also effects measurement of most physical variables and it must be measured for compensation purposes as well.


312 Oct. 2015

Temperature Scale• Celsius, divide the difference between the freezing

and boiling points of water into 100° • Fahrenheit which divide the difference between

the freezing and boiling points of water into 180°• °C = (5 /9) (°F - 32), and °F = (9 /5) °C + 32. • The thermodynamic scale begins at absolute zero,

or 0 Kelvin, the point at which all atoms cease vibrating and no kinetic energy is dissipated.

• 0 K = –273.15° C = –459.67° F. – The official Kelvin scale does not carry a degree sign.

The units are expressed in “kelvins,” not degrees Kelvin.


412 Oct. 2015

Reference Temperatures• No temperature divider or adder• We must rely upon temperatures established by

physical phenomena which are easily observed and consistent in nature.

• The International Temperature Scale (ITS) establishes seventeen fixed points and corresponding temperatures. Examples:– the triple-point (the temperature and pressure at which

solid, liquid, and gas phases of a given substance are all present simultaneously in varying amounts) of water = 0.01C,

– triple-point of hydrogen = -259.3467C, and – freezing point of silver = 961.78C.


512 Oct. 2015

Heat Gain and Heat Loss

• Heat gain:– Environment– Metabolism– Hot food– Shivering

• Heat loss:– Convection– Conduction– Evaporation– IR radiation


612 Oct. 2015

Temperature measuring devices• Temperature can be

measured via a diverse array of sensors. All of them infer temperature by sensing some change in a physical characteristic.

• In the chemical process industries, the most commonly used temperature sensors are thermocouples, resistive devices and infrared devices.

• thermocouples, • resistance temperature

devices– RTD’s and – Thermistors

• infrared radiators, • I.C. sensors, • bimetallic devices, • liquid expansion devices,• change-of-state devices.


712 Oct. 2015

Thermocouples • Two strips or wires made

of different metals and joined at one end.

• Changes in temperature at that junction induce changes in the emf between the other ends.

• As temperature goes up, this output emf of the thermocouple rises, though not necessarily linearly.

Metal A Metal A

Metal B

Metal A

Metal B

+ VAB -

VAB = Seebeck voltage

VAB = T, where , the Seebeck coefficient, is the constant of proportionality. For real world thermocouples, is not constant but varies with temperature.


812 Oct. 2015

Peltier effect

• If a voltage is applied, then there will be temperature change at the junction. This is called the Peltier effect and can be used for heating and cooling (refrigeration).


912 Oct. 2015

Equation of a thermocouple• The output voltage “V” of a simple thermocouple (with a

reference temperature T0 = 0C = 32F) is:

32

31

21

CTBTATV

where T is the temperature of the measuring junction in C, A, B,and C are constants that depend upon the thermocouple material. The sensitivity

volts,

2CTBTATVS

volt/C


1012 Oct. 2015

Characteristics of thermocouples

0

20

40

60

80

500 1000 1500 2000

E

J K

R S T

Temperature, C

Milli

volts

Type of Metals + - E Chromel vs Constantan J Iron vs Constantan K Chromel vs Alumel R Platinum vs Platinum

13% Rhodium S Platinum vs Platinum

10% Rhodium T Copper vs Constantan

Constantan is a metal alloy with %60 copper and %40 nickel


1112 Oct. 2015


1212 Oct. 2015

http://en.wikipedia.org/wiki/File:Thermocouple_circuit.svg

Resistance Temperature Devices• RTD’s R = R0[1 + (T – T0)]• platinum, nickel, or ni alloys

– fine platinum wire wrapped around a mandrel and covered with a protective coating (also abbreviated PRTD).

– most stable temp trans.• Film RTD

– a platinum or metal-glass slurry film is deposited or screened onto a small flat ceramic substrate, etched with a laser-trimming system, and sealed

– device size itself is small, which means it can respond quickly to step changes in temperature.

• Film RTD’s are less stable

• Thermistors – NTC– PTC

• most sensitive temperature transducer

Thermocouple

Thermistor

RTD

Temperature, C

V o

r T


1312 Oct. 2015

Equation of a thermistor

Steinhart-Hart equation:

3)(ln)(ln1 RCRBAT

a simpler equation: CAR

T

)(ln1

)(

00

0

TTTT

eRR


1412 Oct. 2015

http://en.wikipedia.org/wiki/File:NTC_bead.jpg

http://en.wikipedia.org/wiki/File:Thermistor.svg

The Self-Heating Problem

0.1

1.0

10

100

0.10 1.0 10.0 100.0

Current, mA

Vol

tage

, V

+ slope

0 slope

- slope


1512 Oct. 2015

Integrated Circuit (I.C.) Sensors +

To DVM 1 M

1 A/K

To DVM 1 M

1 A/K To DVM

+

10 m

V/K

Current sensor Voltage sensor


1612 Oct. 2015

Temperature

Vol

tage

V

T

Thermocouple

Temperature

Resis

tanc

e

R

T

RTD

Temperature T

Thermistor

Temperature Vol

tage

or c

urre

nt V or I

T

I.C. Sensor

Resis

tanc

e

R A

dvan

tage

s

Disa

dvan

tage

s

Self powered Simple Rugged Inexpensive Wide variety of

physical forms Wide temperature

range

Non-linear Low voltage Reference required Least stable Least sensitive

Most stable Most accurate More linear than thermocouple

Expensive Slow Current source required

Small resistance change

Four-wire measurement

High output Fast Two-wire ohmic measurement

Most linear Highest output Inexpensive

Non-linear Limited temperature range

Fragile Current source required

Self-heating

T < 250 C Power supply

required Self-heating Limited

configurations BME 353 - Biomedical

Measurements and Instrumentation1712 Oct. 2015

Bimetallic Devices

Metal A

Metal B


1812 Oct. 2015

Fluid-Expansion Devices• Types:

– the mercury type: an environmental hazard, so there are regulations governing the shipment of devices that contain it.

– the organic-liquid type.– gas instead of liquid type

• No electric power, do not pose explosion hazards, and are stable even after repeated cycling.

• On the other hand, – they do not generate data that are easily

recorded or transmitted, and – they cannot make spot or point

measurements.

50

0

Safety bulb

Capillary tube

Stem

Temperature sensing bulb


1912 Oct. 2015

Chemical (Change-of-State) Sensors

• Change-of-state temperature sensors – labels, pellets, crayons,lacquers or liquid crystals whose appearance

changes when a certain temperature is reached.– They are used, for instance, with steam traps – when a trap exceeds a

certain temperature, a white dot on a sensor label attached to the trap will turn black.

– Response time typically takes minutes, so these devices often do not respond to transient temperature changes, and accuracy is lower than other types of sensors.

– the change in state is irreversible, except in the case of liquid-crystal displays.

– Even so, change-of-state sensors can be handy when one needs confirmation that the temperature of a piece of equipment or a material has not exceeded a certain level, for instance for technical or legal reasons, during product shipment


2012 Oct. 2015

Radiation Detectors (IR Sensors)


2112 Oct. 2015

Spectral radiant emittance versus wavelength for a blackbody at 300 K on the left vertical axis; percentage of total energy on the right vertical axis.

5

0.001

0.002

0.0030.00312

10 15 20

T = 300 K

m= 9.66 m

25

20

40

60

80

100%

% T

otal

pow

er

Spec

tral r

adie

nt e

mitt

ance

, W-c

m-2·m

m-1


2212 Oct. 2015

10

10

50

100

10

Fused silica

SapphireArsenic trisulfide

Thalliumbromideiodine

Wavelength, m

100

Spectral transmission for a number of optical materials.


2312 Oct. 2015

1 2 3

Wavelength, m

Indium antimonide (InSb)(photovoltaic)

Lead sulfide (PbS)

All thermal detectors

0

20

60

100

4 5 6 7 8

Spectral sensitivity of photon and thermal detectors.


2412 Oct. 2015

EarIR

ShutterAmbient sensor

SensorAmp.

MUX A/D

Shutterswitch

WindowWaveguide

Microprocessor

Digitaldisplay

TaTb

The infrared thermometer opens a shutter to expose the sensor to radiation from the tympanic membrane.


2512 Oct. 2015

Details of the fiber/sensor arrangement for the GaAs semiconductor temperature probe.


2612 Oct. 2015

Documents

Temperature Measurements Principles Measuring Devices Applications بِسْـــــمِ اللهِ الرَّحْمَنِ الرَّحيـــــِمِ